Cathode-ray tube having conductive internal coating comprised of iron oxide and graphite

ABSTRACT

A cathode-ray tube having a conductive internal coating comprised of iron oxide, graphite, and an alkali silicate on the interior walls of the tube.

United States Patent 1191 'Maley et al.

' [451 Feb. 12,1974

CATHODE-RAY TUBE HAVING CONDUCTIVE INTERNAL COATING COMPRISED OF IRONOXIDE AND GRAPHITE Inventors: James John Maley, Lancaster;

Donald Walter Bartch, Columbia, both of Pa.

Assignee: RCA Corporation, New York, NY.

Filed: Nov. 26, 1971 Appl. No.: 202,428

us. 01. 220/2.1 A, 117/33.3, 117/335 c, 117/54,117/124 A, 117/211,117/226,

v 313/39, 313/92 .'1m.c1. H05b 33/23, c09 1/44 Field of Search...117/54, 37 R, 124 A, 33.5 c, 117/333, 226, 211; 252/313 s; 313/89, 92;220/2.1 A

Primary Examiner-Alfred L. Leavitt Assistant Examiner-l4. F. Es o sitoAttornej Agent, or F irm-G. H. Bruestle; L. Greenspan ABSTRACT Acathode-ray tube having a conductive internal coating comprised of ironoxide, graphite, and an alkali silicate on the interior walls of thetube.

7 Claims, 1 Drawing Figure CATHODE-RAY TUBE HAVING CONDUCTIVEINTERNAL'COATING COMPRISED OF IRON OXIDE AND GRAPHITE BACKGROUND OF THEINVENTION This invention relates to an improved cathode-ray tube havingan internal coating comprised of iron oxide and graphite on the interiorwalls of the tube/ Cathode-ray tubes usually have a conductive internalcoating on the interior walls of the tube. This'coating is used to carrya high potential of the order of to 30 kilovolts, which is applied atthe anode button. The most common internal coating consists essentiallyof particulate graphite and an alkali silicate binder. The coating isapplied to the walls of the tube by spraying and/or brushing as anaqueous .coating composition, then dried, and then baked in air at about400C for one hour. After baking, this prior-art coating has apoint-to-point resistance of about 50 to 90 ohms per inch. During normaltube processing and in subsequent handling, graphite particles arereleased from the coating and may cause excessive arcing and electricalleakage in the electron gun during the operation of a finished tube.Excessive arcing and electrical leakage are causes of poor high voltagestability of the tube.

SUMMARY OF THE INVENTION ent to glass, releases fewer graphite particlesboth because there is a lower content of graphite particles present andbecause the coating is more cohesive, and is more scratch resistant.And, surprisingly, loose'iron oxide particles have been shown not to bedetrimental to the high voltage stability of the tube. The coating hasan adequate conductivity and is otherwise compatible in performance withprior coatings. In addition, the internal coating may be applied from anaqueous coating composition by presently used techniques.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a partiallybroken-away longitudinal view of a novel cathode-ray tubeof theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Example The cathode-ray tubeillustrated in the sole FIGURE is an apertured-mask-type kinescope. Thetube includes an evacuated envelope designated generally by the numeral21, which includes a neck 23 integral with a funnel 25, and a faceplateor panel 27 joined to the funnel 25 by a seal 29, preferably of adevitrified glass. There is a luminescent layer 31 of a phosphormaterial on the interior surface of the faceplate 27. There is alight-reflecting metal coating 33, as of aluminum metal, on theluminescent layer 3I. The luminescent layer 31, when suitably scanned byan electron beam or beains from a gun in a mount assembly 35 located inthe neck, 23, is capable of producing a luminescent image which may beviewed through the faceplate 27.

There is an electrically-conductive internal coating 37, consistingessentially of an alkali silicate binder material, iron oxide particles,and carbon particles, on a portion of the interior surface of the funnel25 between the mount assembly 35 and the seal 29. Three metal fingers 39space the mount assembly 35 from the neck wall and connect the forwardportion of the mount assembly 35 with the internal coating 37.

Closely spaced from the metal coating 33 toward the mount assembly 35 isa metal mask 41 having a multiplicity of apertures therein. The mask 41is welded to a metal frame 43 which is supported by springs 47, whichare attached to the frame 43, on studs 45 integral with the panel 27.Inasmuch as the invention is con cerned primarily with the conductiveinternal coating 37, a detailed description of the components and partsnormally associated with the neck and faceplate 23 and 27 is omitted orshown schematically.

The tube of this example may be fabricated by methods known in the art.The mask 41, frame 43 and springs 47 are assembled. The luminescentlayer 31 and the metal coating 33 are deposited on the inner surface ofthe faceplate 27. The conductive internal coating 37 is applied to theinterior surface .of the funnel 25 and an adjacent portion of the neck23 as indicated in the sole FIGURE. The internal coating 37 is appliedfrom an aqueous suspension in two steps. First, a portion is brushed onin the neck 23 and the adjacent portion of the funnel 25, and then aportion is sprayed on in the funnel 25, so that the sprayed-on portionoverlaps the brushed-on portion. A typical coating formulation(44478/36) is as follows:

, 100 grams graphite percent below l0 microns) 220 grams ferric oxide FeO (90 percent below 10 microns) 300 grams aqueous solution containing 54weight percent sodium silicate (Na ozSiO is about l:2.0)

2.2 grams dispersant 500 grams deionized water This formulation has aviscosity of about 20.5 seconds. After application, the coating is driedin air.

Next, a bead of devitrifying glass frit is deposited on the seal land ofthe funnel 25. The frame 43 with the mask 41 and springs 47 attachedthereto is mounted on the studs 45. The seal land of the panel 27 isplaced against the bead of frit on the seal land of the funnel 25,

and the assembly is heated at about 400C until the frit melts anddevitrifies and forms a vacuum-tight seal 29 between the panel 27 andthe funnel 25. During the heating to form the seal 29, the heat alsobakes the funnel coating 37, driving out moisture and rendering thefunnel coating 37 electrically-conductive and chemically-stable to theatmosphere. After cooling to room temperature, the funnel coating 37has, by the tests described below, a hardness of about l,200 grams, ascratch resistance of about 6,968 particles, mostly iron oxide, and anelectrical resistance of about 200 ohms per inch.

The metal fingers 39, the electron gun and the convergence assembly areassembled on a glass stem producing the mount assembly 35. The metalspacer fingers 39 are depressed and inserted into the neck 23 and slidinto the tube until the mount assembly 35 is at the desired position.The fingers 39 slide on a portion of the coating 37 during this step andmay abrade some of the coating material therefrom. Next, the glass stemis sealed to the neck 23. Finally, the entire tube is baked at about440C, the tube exhausted and then sealed.

GENERAL CONSIDERATIONS The internal coating may be used in anycathode-ray tube including picture tubes, display tubes, oscilloscopes,camera tubes and storage tubes. The coating is particularly suitablewhere an adherent conductive coating is desired on the interior glasssurfaces of the tube. The conductive internal coating is prepared byapplying to the interior surface of the glass funnel 25 a coating of anaqueous suspension ofa desired compo sition, drying the coating and thenbaking the coating at about 200 to 450C. The suspension comprises about200 to 600 weight parts iron oxide particles, 100 weight parts powderedgraphite and about 50 to 150 weight parts alkali silicate solids as anaqueous solution. Funnel coatings for use in kinescopes according to theinvention preferably consist essentially of about 100 weight partsgraphite, about 200 to 220 weight parts dehydrated ferric oxide andabout 125 to 150 weight parts sodium silicate solids. A dispersant andadditional water are added to adjust the coating qualities of thecoating formulation as desired. The iron oxide is in anhydrous form andmay be any oxide of iron, such as ferric oxide Fe O ferrous oxide FeO,or ferrosoferric oxide Fe O The preferred form is ferric oxide Fe O Theparticle-size range of the iron oxide is such that 90 percent is belowmicrons. The graphite may be any of the varieties that can be used formaking conductive coatings. The particle-size range of the graphite issuch that 90 percent of the graphite is below 10 microns. The alkalisilicate solids are in an aqueous solution of about 32 to 63 weightpercent solids. The alkali is preferably sodium, but may be potassium orlithium. Where sodium silicate is used, the ratio of alkali to silica inthe silicate may be in the range of 1:1.6 to 1:3.8.

The constituents of the suspension are mixed together preferably withabout 0.1 to 0.3 weight percent of dispersant such as Marasperse,marketed by American Can Company, New York, New York. The constituentsof the coating formulation are mixed together and then ball milled for aperiod of time, for example about 6 hours. Changing the proportions ofiron oxide, graphite and alkali silicate affects the electricalresistance of the final coating. Increasing the proportions of ironoxide increases the electrical resistance of the finished coating.Increasing the proportions of alkali silicate increases the electricalresistance and scratch resistance of the finished coating. Increasingthe proportions of graphite decreases the electrical resistance anddecreases the scratch resistance of the finished coating. However, theinternal coatings described herein provide a set of practicalcompromises of the electrical and physical properties needed for theiruse in cathode-ray tubes.

The coating formulation may be applied by any convenient process. It ispreferred to brush the coating onto the interior wall of the neck 23 sothat a sharply defined edge is produced opposite the mount assembly 35.It is preferred to spray the coating formulation onto the interiorsurface'of the funnel 25, overlapping the brushed-on portion. Sprayingis a rapid process for covering the relatively large funnel area. Eitherair or airless spraying can be used.

The physical and electrical properties of the iron oxide-graphiteinternal coating employed in the novel tube and similar priorall-graphite coatings used in prior tubes have been measured many times.The results are quite consistent. A typical set of comparative data,along with the formulations, is shown below in the Table. The particularmaterials used are those mentioned in the Example.

When a prior-art all-graphite coating is abraded (as with bulb spacersor with the getter housing), graphite particles are released which havebeen shown to be detrimental to high voltage stability, such as causingexces-v sive arcing or electrical leakage. Far fewer particles aregenerated from the iron oxide-graphite coating described herein. Thisfact in itself results in better high voltage stability. In addition,since the coating is mainly iron oxide, most of the particles which arereleased from abrasion are iron oxide. Unlike graphite particles, looseiron oxide particles have been shown not to be detrimental to the highvoltage stability of the tube. For these reasons, much better highvoltage electrical stability is obtained with the novel tube. Thus, thepresence of iron oxide in the internal coating provides adequateelectrical properties, improved abrasion and scratch resistance and,when particles are released, produces less arcing and electricalleakage.

In a color kinescope, for example, electrons are projected to the screenand are then returned to the high voltage supply through the anodebutton. If the resistance of the internal funnel coating is too high 1 X10 ohm), there will be a significant voltage drop between the appliedvoltage at the anode button and the screen. This will be observed as adull picture on the screen. It may be argued that a high resistancecoating 1 X 10 ohm) may still be used with a conducting band between thescreen and the anode button to prevent voltage drop. In principle,.thisis true, but it is difficult to produce a reproducible coating withsuitable propertiesin practice. An additional coating must be used,which is undesirable in production. Also, when a high resistance coatingis used over the interior surface of the funnel, the resistance of thecoating must be uni form. If isolated patches of insulating coating arepresent, there will be nonuniform charging of the funnel coating. Thiswill have an undesirable effect on the electron beam and will probablybe observed as convergence drift or dynamic convergence errors. Highresistance internal coatings with uniform resistances are difficult toprepare. The electrical resistance of the iron oxide-graphite coatingdescribed herein is about 200 to 2,000 ohms per inch, and preferablyabout 200 to 300 ohms per inch. Consequently none of the problemsassociated with high resistance coatings are present. In the novel tube,the resistance of the oxide-graphite coating is low enough so that noother coating is required, and no electrical problems result from theuse of this coating. In addition, the high voltage stability of thekinescope is greatly enhanced.

TABLE Fe o -Graphite All-Graphite Iron Oxide 200 to 600 grams 0 GraphitelOO grams l00 grams Silicate Solids 50 to I50 grams lOO grams Dispersant1.0 to 4.4 grams 3.2 grams Water 400 to 1,500 grams 275 grams Viscosity20 to 222 secs. l7.5 vto 20 secs. Hardness 1,000 to 1,250 25 to 50 gramsgrams Coating Particle 2.5 to 3.0 p. 3.0 .1. Size (Median) Adherence Novisible par- Particles on over ticles 50% of the tape Scratch Resis- X=7,000 par- Y 20,000 partance ticles (mostly ticles (all iron oxide)graphite) Electrical Rc- 200 to 2,000 50 to 90 ohms] sistance ohms/inchinch l. Hardness is measured by a Hoffman Method test using a weightedcylindrical stylus. The reading obtained is the weight necessary toscratch the coating so that the substrate is visible. This indicates theamount of coating which may be scraped off when the coating is abraded;e.g., when the mount assembly is inserted into the neck. Generally, theharder the coating, the better it is for fabrication and operation.

2. Adherence is measured by a Scotch Tape Test. A 2000-gram cylinder isrolled over a piece of tape on the surface of the funnel coating,pressing the pressure-sensitive adhesive against the funnel coating. Thetape is then removed. and a visual inspection is made of the particleson the tape. The number of particles on the time is a relative measureof particles loosely adhereing to the coating surface. These particlesmay be removed from the funnel coating without abrasion during theprocessing or life of the tube. Generally, the fewer the number ofparticles on the tape, the better the coating is for fabrication andoperation.

3. In the scratch re si tag tg tg, a gleanlqu nt is inserted into acoated glass neck to a distance of 1 inch. The mount is withdrawn, andthe coated neck and the gun are washed with freon. The freon isfiltered, and the particles are collected on the filter paper. Theparticles are then counted by aid of a microscope. This test is ameasure of the number of particles that may be released from the funnelcoat ing during mount insertion. In tube construction, a significantnumber of these particles fall on the mount and cause arcing andelectrical leakage. The probability of arcing and electrical leakage inthe tube increases with the relative increase in the number of particlesproduced during this ten the coating consists essentially of about [00weight pans graphite particles, 200 test, using the same mount.

4. in the electrical resistance test. two -inch-diameter metal ballsmounted a known distance apart in'an insulating plate are appliedagainst the coating, and the DC resistance between them is measured. 7

We claim:

1. ha cathode-ray tube comprising an evacuated envelope including afunnel, the improvement comprising a conductive coating adhered to atleast a portion of the interior walls of said funnel, said coatingconsisting essentially of graphite particles, iron oxide particles andan alkali silicate binder, the weight ratio of iron oxide particles tographite particles being in the range of 2 to l and 6 to I, said coatingbeing of such thickness as to provide a point-to-point electricalresistance of about 200 to l0,000 ohms per inch.

2. The tube defined in claim 1 wherein the coating consists essentiallyof about weight parts graphite particles, 200 to 600 weight parts ironoxide particles and about 50 to 150 weight parts alkali silicate binder.

3. The tube defined in claim 2 wherein said alkali silicate binder issodium silicate.

4. In a television picture tube comprising an evacuated glass envelopeincluding a neck for housing an electron gun, a faceplate panel oppositethe neck, and a funnel providing a closed transition between said neckand said panel, the improvement comprising a coating adhered to at leasta portion of the interior walls of saidfunnel and neck, said coatingconsisting essentially of about 100 weight parts graphite particles,about 200 to 600 weight parts iron oxide particles and about 50 to 150weight parts alkali silicate binder, said coating being of suchthickness as to provide a point-topoint electrical resistance of about200 to 2,000 ohms per inch.

5. The tube defined in claim 4 wherein said electrical resistance isabout 200 to 300 ohms per inch and the iron oxide is ferric oxide.

6. The tube defined in claim 5 wherein said coating consists essentiallyof about 100 weight parts graphite particles, about 200 to 220 weightparts iron oxide particles, and about to weight parts alkali silicatebinder.

7. The tube defined in claim 6 wherein said alkali silicate is sodiumsilicate.

UNITED STATES PA'IENT OFFICE (IERTIFICA'IE ()F CORRECTION 'PA'ILN'I NO.3, 791, 54 DATED February 12, 1974 INVENT()R(S) 1 James John Marley andDonald Walter Bartch H is certified that error appears in the aboveidentified patent and that said Letlers Patent are hereby corrected asshown below:

Column 6, line 9 change "10,000" to -2,000

Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents and Trademarks RUTH C. MASONAttesting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONQ Patent No. 3,791,546 Dated February 12, 1974 Inven-ltofls) James JohnMaley and Donald Walter Bartch It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 24 after "of' change "a" tow-thew Column 5, line 26change .tpae" to -tape+ Column lines 38 remove "ten the eo ati ng consists essentiand 39 ally of abOut ljOO weight. parts graphite particle's,ZOO" Signed and sealed this 18th day of June 1971;.

(SEAL) Attest:

EDWARD MQFLETCHERJRQ I c'. MARSHALL DANN I Attesting Officer I vCommissioner of Patents FORM F'O-IOSO (10-69) I sc -nc 503754359 e vs.sovznuuzm PRINTING OFFICE: 19' o-ass-a 4

2. The tube defined in claim 1 wherein the coating consists essentiallyof about 100 weight parts graphite particles, 200 to 600 weight partsiron oxide particles and about 50 to 150 weight parts alkali silicatebinder.
 3. The tube defined in claim 2 wherein said alkali silicatebinder is sodium silicate.
 4. In a television picture tube comprising anevacuated glass envelope including a neck for housing an electron gun, afaceplate panel opposite the neck, and a funnel providing a closedtransition between said neck and said panel, the improvement comprisinga coating adhered to at least a portion of the interior walls of saidfunnel and neck, said coating consisting essentially of about 100 weightparts graphite particles, about 200 to 600 weight parts iron oxideparticles and about 50 to 150 weight parts alkali silicate binder, saidcoating being of such thickness as to provide a point-to-pointelectrical resistance of about 200 to 2,000 ohms per inch.
 5. The tubedefined in claim 4 wherein said electrical resistance is about 200 to300 ohms per inch and the iron oxide is ferric oxide.
 6. The tubedefined in claim 5 wherein said coating consists essentially of about100 weight parts graphite particles, about 200 to 220 weight parts ironoxide particles, and about 125 to 150 weight parts alkali silicatebinder.
 7. The tube defined in claim 6 wherein said alkali silicate issodium silicate.